Conventionally, in order to sense abnormality of a structure that generates atmosphere of high temperature, such as a conduit or a valve of a plant (e.g. a nuclear power plant), or an internal combustion engine, a sensor is put inside the structure. Examples of the sensor are as follows; an acoustic emission sensor for sensing an acoustic emission that is an elastic wave generated when a crack or a crevice is made, and a piezoelectric vibration sensor for sensing information of an abnormal vibration or acceleration. As the examples of these sensors, a compressed type, a cantilevered type, a diaphragm type, and a shearing type are known.
Out of these sensors, a thin-filmed piezoelectric sensor (compressed type) includes a laminated body in which a pedestal, an electrode on the pedestal, a piezoelectric device, an electrode on a load substance, and the load substance are laminated successively, is used so that a bottom side of the pedestal is firmly installed on a target of measurement. When the target of measurement vibrates, the vibration is conducted to the pedestal of the sensor. The pedestal of the sensor vibrates with the target of measurement, but a vibration of the load substance gets behind due to an inertial power of the load substance, and a compressive stress or a tensile stress is generated in the piezoelectric device in proportion to the vibration acceleration. And an electric charge or voltage in proportion to the stress is generated on both sides of the piezoelectric device, and an electric current flows between the two electrodes located on both sides of the piezoelectric device, and by sensing the amount of the electric current, the amount of a vibration and acceleration of the target of measurement can be sensed.
Usually, this kind of piezoelectric sensors uses a piezoelectric device made of a piezoelectric material such as lead titanate zirconate and polyvinylidene fluoride, but the piezoelectric device made of such a piezoelectric material has low Curie temperature at which polarization vanishes, and maximum temperature at which the device can be used is about 300° C. at most. Therefore, in order to keep a piezoelectric device at suitable temperature, for example, Japanese Laid-Open Patent Publication No. 203665/1993 (Tokukaihei 5-203665) discloses an arrangement for cooling a piezoelectric device by use of a Peltier element. But the function of a Peltier element is limited to generating local temperature gradient, so that the sensor cannot be applied to a place where a cooling device cannot be installed on the outside and temperature rises high on the whole.
Further, in the case of a vibration such as acoustic emission, the vibration may attenuate due to characteristics of a vibration-transmitting material on the way, or an unnecessary vibration may mix from the outside while the vibration is transmitted, and therefore it is desirable to measure the vibration as near as possible to the place where the vibration has been generated. However, as for a structure with its temperature rising high, as shown above, a usual thin-filmed piezoelectric sensor is inferior in heat-resistance, and so the vibration is to be induced to a distant place at low temperature by a vibration-transmitting stick and measured there. But in this case, due to reasons such as attenuation of a vibration or mixture of a noise, the vibration of the target of measurement cannot be measured accurately.
Therefore, in order to make a thin-filmed piezoelectric sensor that has heat-resistance, the use of, for a piezoelectric layer, a piezoelectric material with high Curie temperature such as lithium niobate is disclosed, for example, on Japanese Laid-Open Patent Publication No. 34230/1993 (Tokukaihei 5-34230). The Curie temperature of lithium niobate is about 1140° C. and measurement at high temperature is possible without cooling means. But lithium niobate has several faults; it is difficult to be formed as a thin film, it cannot gain piezoelectric characteristics unless it is single crystal, and it is difficult to be fabricated or processed and costs a lot.
Further, in a case of a thin-filmed vibration sensor for high temperature, disclosed on Japanese Laid-Open Patent Publication No. 122948/1998 (Tokukaihei 10-122948), in order to solve the problems, zinc oxide or aluminum nitride is used as piezoelectric ceramic without Curie temperature, and a thin film made by orientating zinc oxide or aluminum nitride in parallel to a c-axis is used as a piezoelectric device. A thin film made of aluminum nitride or zinc oxide is a hopeful material for downsizing a piezoelectric device.
However, in order to use aluminum nitride or zinc oxide as a piezoelectric device, a thin film in which aluminum nitride or zinc oxide is dipole-orientated is needed, and in order to improve the piezoelectric characteristics of the piezoelectric device, the degree of its dipole-orientation needs to be increased. However, though thin films made of aluminum nitride or zinc oxide have been made in many processes until now, studies on the degree of dipole-orientation of a thin film made of aluminum nitride or zinc oxide have been hardly done, and therefore there is a problem that it is difficult to fabricate a thin film in which the degree of the dipole-orientation is controlled.
Further, the thin film made of aluminum nitride or zinc oxide has a very large internal stress created in a film forming process, and therefore, for example, in the case of forming the thin film on an electrode made on a substrate, there is a problem that a crack appears in the electrode or the thin film peels off from the substrate, accompanied by the electrode.
The present invention is proposed to solve the problem, and its purpose is to provide an efficient piezoelectric device that uses a thin film with a high degree of dipole-orientation and is made of aluminum nitride or zinc oxide, and to provide a method for fabricating the efficient piezoelectric device, with easiness and in a low price.